Field of the Invention
This invention relates to the coating of substrates, and in particular to a process for more effective and rapid drying of polymer layers.
Description of the Related Art
A continuing trend in semiconductor technology is the formation of integrated circuit (IC) chips having more and faster circuits thereon. Such ultralarge scale integration has resulted in a continued shrinkage of feature sizes with the result that a large number of devices are made available on a single chip. With a limited chip surface area, the interconnect density typically expands above the substrate in a multi-level arrangement and the devices have to be interconnected across these multiple levels. The interconnects must be electrically insulated from each other except where designed to make contact. Usually electrical insulation requires depositing dielectric films onto a surface, for example using a CVD or spinning-on process. The shrinkage in integrated circuit design rules has simultaneously reduced the wiring pitch. These have made the signal propagation delay in the interconnects an appreciable fraction of the total cycle time. The motivation to minimize signal delay has driven extensive studies to develop a low dielectric constant (low-k) material that can be used as an inter-level dielectric in integrated circuit (IC) manufacturing. The majority of low-k materials used in the ILD layer are based on thermally cured spin-on organic or inorganic polymers.
Polyimide is a polymer material often used in the production of semiconductor substrates such as silicon wafers. Polyimide is a desirable insulating material for semiconductor wafers because of its outstanding physical properties. Unfortunately, polyimide typically requires a long time to cure when conventional heating techniques are used. A cure cycle of several hours is typical and this often becomes the pacing step in semiconductor fabrication. In addition, there are other problems involved with curing polyimide resin with conventional heat. For example, when polyimide resin is cured in a conventional furnace, the outer surface of the resin typically cures faster than the center portions. This can cause various physical defects, such as the formation of voids, and can result in inferior mechanical properties such as reduced modulus, enhanced swelling, solvent uptake, and coefficient of thermal expansion.
A polyimide precursor may be applied to a substrate, and then dried to prepare for imidization of the polymer. A goal of the drying process is to remove the solvent from the polymer (which may be N-Methyl-2-pyrrolidone, NMP, for example), and it can also be important to remove oxygen during the drying process. In addition, further goals of the drying process are to minimize or eliminate any bubbles/voids in the polymer layer, to minimize discoloration to the layer that may be induced by heating, and to fully remove residual solvent from the precursor mix. Each of these items sought to be eliminated may interfere with subsequent process steps, or enhance the probability of failure of a device containing the polyimide layer.
In addition to need to dry polyimide precursor layer, some semiconductor manufacturing processes present the need to outgas plastic mold compounds. For example, in Fan Out Wafer Level Processing (FOWLP), the plastic molding compounded which the semiconductor die are molded into may require outgassing to remove compounds that will interfere with subsequent processing, such as metallization steps.
What is called for is a process which can quickly dry a polymer compound, which may be a polyimide precursor. What is further called for is such a process that minimizes or eliminates void, discoloration, and that also results in thorough solvent removal. What is also called for is a process which can both quickly dry a polyimide precursor and outgas plastic molding in a single step, greatly reducing processing time.